Sheet containing device

ABSTRACT

In a sheet conveying device comprising at least one conveying arrangement with a porous conveyor belt that can be moved in a circulating manner, at least one vacuum source and at least one air line whose one end is connected to the vacuum source and whose other end abuts against the porous conveyor belt, a simple and automatic control of the suction pressure for different sheets is achieved by at least one constant pressure controller in the air line in order to essentially maintain constant a pressure differential between the interior space of the air line and its environment.

TECHNICAL FIELD

The present invention relates to a sheet conveying device, said device operating with a vacuum.

Such sheet conveying devices are used at various locations in machines for printing and other processing of individual sheets. For example, a vacuum is used in such a sheet conveying device in order to attract individual sheets by suction off a stack and to then feed the individually attracted sheets to the subsequent processing operation.

BACKGROUND OF THE INVENTION

Usually, such a sheet conveying device comprises a porous conveyor belt for transporting a sheet. An air line with a prevailing internal vacuum is arranged adjacent to the conveyor belt in such a manner that air is drawn by suction through the conveyor belt. The vacuum in the air line is generated, for example, by a blower. By sucking air though the conveyor belt, a sheet may be sucked against the conveyor belt and then be transported away by said conveyor belt. If there is no sheet abutting against the conveyor belt, air is continuously sucked through the conveyor belt, so that, as a rule, there is only a minimal vacuum in the air line. This initial vacuum, i.e., the pressure differential with respect to the environment, thus substantially depends on the suction power of the blower and the flow resistance of the porous conveyor belt. If a sheet reaches a specific distance from the porous conveyor belt, it is attracted in dependence of the vacuum level and the properties (of the material) of said sheet. This combination of the distance from the conveyor belt, the level of the vacuum and the properties of the sheet at the time when said sheet is being attracted by suction is also referred to as the dynamic work point of the sheet conveying device. When the sheet has been sucked against the porous conveyor belt, said belt is covered by the attracted sheet, which causes the flow resistance of the conveyor belt to be significantly increased. Now, the air line can hardly attract any air through the conveyor belt. As a result of this, the pressure in the air line drops because the blower continues to draw air through the air line.

The dynamic work point of the sheet conveying device, as stated, depends on the sheet and its properties, among other things. For example, as a rule, using the same vacuum, a light-weight flexible sheet may be attracted over a greater distance than a heavy inflexible sheet. But also the normal forces that are necessary for separating superimposed sheets of a stack may vary for different sheets. Even sheets of the same material—e.g., with different fiber orientation that affects the stiffness of the sheet—may result in different dynamic work points. Inasmuch as the sheet properties are prespecified by the sheet, the dynamic work point must now be adjusted by way of the distance and/or the vacuum in such a manner that a reliable attraction of the sheets takes place. As a rule, an exact adjustment of the distance is difficult to achieve and, usually, a distance range is provided within which the sheets must be attracted by suction. To ensure that this is the case for all sheets, one possibility would be to simply increase the vacuum to a level sufficient to ensure the attraction of the sheet. However, this may result in the problem that, in particular, light-weight flexible sheets are damaged because the pressure in the air line drops as soon as the sheet has been drawn against the conveyor belt. As a result of this, also the forces acting on the sheet are increased, thus potentially damaging the sheet. Consequently, only a limited adjustment of the dynamic work point is possible by adjusting the level of the vacuum. Frequently, it is possible to only use sheets satisfying specific specifications in known sheet conveying devices in a machine, i.e., the sheets must maintain a prespecified undulation, stiffness, density, thickness and fiber orientation in order to be able to be processed by a machine.

SUMMARY OF THE INVENTION

Therefore, it is the object of the present invention to provide a sheet conveying device that can be used for a broad spectrum of sheets.

This object is achieved by a sheet conveying device in accordance with claim 1, as well as by a method for separating sheets in accordance with claim 32. Additional embodiments of the invention are obvious from the subclaims.

In particular, the sheet conveying device comprises a conveying arrangement with a porous conveyor belt that can be moved in a circulating manner, at least one vacuum source, and at least one air line, the one end of said air line being connected to the vacuum source and the other end of said air line abutting against the porous conveyor belt. Furthermore, at least one constant pressure controller is provided in the air line in order maintain a substantially constant pressure differential between the interior space of the air line and its environment. This way, a broad spectrum of sheets may be transported because any excessive rise in the vacuum in the air line following the attraction of a sheet is avoided. Thus, the vacuum that originally prevails in the air line when no sheet is attracted may be selected to be relatively high.

Advantageously, the conveyor belt is arranged so as to circulate around at least two rollers in order to achieve a simple design.

Preferably, the air line contains—downstream of the constant pressure controller—a main flap in order to partially or completely seal the air line with respect to the vacuum source. Consequently, a vacuum may be preset and a pressure loss avoided.

Preferably, the air line comprises a main line that is connected, at its one end, to the vacuum source, and comprises a plurality of branch lines. Due to this branching, one vacuum source is sufficient for several purposes of use. Advantageously, each of the plurality of branch lines leads to a conveying arrangement in order to be able to supply several conveying devices with one vacuum source. One constant pressure controller is advantageously arranged in the region of each branch line in order to permit an individual adjustment of the vacuum. Preferably, the main flap is arranged so as to close the branch line with respect to the main line in order to avoid pressure losses via a sheet conveying arrangement that is not in use. In this case, the constant pressure controller is advantageously arranged between the main flap and the free end of the branch line.

Preferably, the constant pressure controller is in an open position when a pressure differential between the interior space of the air line and its environment is greater than a threshold value, thus limiting the pressure differential to a prespecified value.

Preferably, the constant pressure controller comprises a valve element and a spring that holds the valve element in a closed position in order to achieve a design that is as simple as possible. The spring displays—as one option—a spring characteristic that is variable over the spring path in order to generate a changed response behavior. Furthermore, the spring bias can advantageously be adjusted to provide adjustability at the location of use.

Alternatively, the sheet conveying device may comprise a pressure sensor in order to be able to detect the internal pressure in the air line. At least one pressure sensor may be arranged in each branch line in order to be able to determine the internal pressure in the branch line. In this case, the sheet conveying device may further comprise a controllably driven pressure control valve and a control device that communicates with the pressure sensor and the pressure control valve in order to control an actuation of the pressure control valve. In this way, it is possible to control the opening behavior as desired.

Advantageously, the constant pressure controller comprises either a flap valve or a sliding valve in order to achieve a simple design. In particular for the sliding valve, the opening cross-section of the sliding valve preferably does not increase proportionally along its opening path.

Preferably, the sheet conveying device is arranged in a sheet separating device and/or in a feeder of a printing machine. As a result of this, a broad spectrum of sheets may be fed to the printing machine.

At least a part of the conveying device can be advantageously moved toward the sheet support and away therefrom in order to be able to lift a sheet off the stack of sheets on the sheet support.

The sheet support of the sheet separating device is advantageously adjustable as to height in order to be able to accommodate various sheet stacks and sheets and to advance them during continuous use.

Preferably, the movable part of the conveying arrangement is the at least one conveyor belt. Consequently, the sheet in direct contact with the conveyor belt is transported away. Alternatively, the part of the conveying device is at least one suction finger connected to the vacuum source, thus requiring less mass to be moved. Alternatively, the part of the conveying device may be at least one of the rollers, thus requiring less mass to be moved. Alternatively, the sheet separating device can be moved, as a whole, toward the sheet support and away therefrom in order to be able to lift a sheet off the stack of sheets on the sheet support.

If it exists, the suction finger—in its position away from the sheet support—is preferably separated from the vacuum source in order to avoid damaging the sheet as it is being transported away by the conveyor belt. The connection of the suction finger with the vacuum source is preferably open toward the environment when said suction finger is in its position remote from the sheet support, thus providing a simple and fast pressure shut-off.

Advantageously, the part of the conveying arrangement can be moved in a linear or pivotable manner in order to achieve a simple design.

In accordance with the invention, a method for separating sheets in a sheet separating device is also provided, wherein a stack of sheets is located below a sheet feeding device operating with a vacuum, wherein the uppermost sheet of the stack of sheets is attracted by means of a vacuum, and wherein, following the attraction of the sheet, the level of the vacuum with which the sheet is being attracted is limited via a constant pressure control valve inside the sheet separating device to a prespecified value. With this method, the aforementioned advantages can be achieved; in particular, this method permits the use of an additional format range of the sheets to be conveyed.

Preferably, the method additionally comprises the step of moving at least a part of the sheet separating device into the vicinity of the uppermost sheet of the stack of sheets before attracting the uppermost sheet by vacuum, and the step of moving said part of the sheet separating device away from the stack of sheets after attracting the uppermost sheet, followed by the step of transporting the sheet away. This makes it possible to reduce the distance range over which the sheets must be attracted by suction, as a result of which the spectrum of sheets that can be conveyed is additionally increased.

The step of moving at least a part of the sheet separating device preferably comprises the step of moving at least one suction finger operating with a vacuum in order to permit a rapid motion. In so doing, the vacuum at the suction finger is shut off, preferably after the step of moving the sheet away from the stack of sheets, in order to avoid damage to the sheet.

The step of moving at least a part of the sheet separating device alternatively comprises the step of moving at least one porous conveyor belt in order to bring the sheet into direct contact with the conveyor belt.

Alternatively, the step of moving at least a part of the sheet separating device comprises the step of moving the entire sheet separating device in order to avoid any mutual displacement of the individual components. The steps of moving the part of the sheet separating device away from the stack of sheets and of transporting away the sheet preferably occur at the same time in order to permit rapid processing of the sheets.

The step of moving at least a part of the sheet separating device comprises either a linear movement or a pivoting movement.

Hereinafter, the invention, as well as additional details and advantages thereof, will be described in greater detail with the use of preferred embodiments and with reference to the drawings.

They show in

FIG. 1 a schematic side view of a sheet conveying device in accordance with a first embodiment;

FIG. 2 a schematic side view of a sheet conveying device in accordance with a second embodiment;

FIG. 3 a schematic side view of a sheet conveying device in accordance with a third embodiment.

In the description hereinafter, the terms right, left, up, down, as well as corresponding expressions are used with reference to the figures; however, they should not be understood to have a restrictive meaning. Furthermore, the same reference signs are used in the drawings and in the description to describe the same or corresponding parts and features in different embodiments. It should be noted that the features of different embodiments may be freely combined, provided they are compatible.

FIG. 1 shows a first embodiment of a sheet conveying device 1 that comprises a conveying arrangement 3, a blower 5, an air line 7 and a constant pressure controller 9.

Located under the sheet conveying device 1 is a sheet support 11 on which a sheet stack 12 of individual sheets 13 is arranged. The sheet conveying device 1 is provided to convey individual sheets 13 away from the sheet stack 12. The sheet conveying device 1 operates as a sheet separating device for removing and trans-porting individual sheets 13 away from the sheet stack 12. The sheet conveying device 1 is arranged in a not-illustrated feeder of an also not-illustrated printing machine, said feeder being disposed to introduce sheets into a printing process. However, the sheet conveying device 1 may also be arranged at a suitable location of another sheet processing device. As will be explained in greater detail hereinafter, it is possible to move at least a part of the sheet separating device 1 toward the sheet support 11 and away therefrom. As a result of this, a distance between the conveying arrangement 3 and the uppermost sheet 13 of the sheet stack 12 on the sheet support 11 can be bridged quickly. For this, a pivoting movement as well as a linear movement are used.

The sheet support 11 is height-adjustable and lifts the sheet stack 12 in the vicinity of the conveying arrangement 3. Usually, air is blown (from the right, in the figures) laterally at the sheet stack 12 in the upper region of a not-illustrated pressurized air nozzle that is also referred to as an “air knife” in order to slightly fan out the sheets 12 before they are being received by the sheet conveying device 1.

The conveying arrangement 3 comprises a porous conveyor belt 15 that is passed around two rollers 17; 18. At least one of the rollers is connected with a not-illustrated motor. The motor is provided so as to intermittingly drive the roller in a controlled manner. As an alternative, the porous conveyor belt 15 may consist of a plurality of porous conveyor belts arranged next to each other (perpendicular to the sheet plane of the figures), said conveyor belts being movable so as to circulate around the rollers 17, 18. Depending on the format range of the sheets 13 to be conveyed, the width of the conveyor belt 15 varies, or more or fewer conveyor belts are arranged next to each other. It should be noted that in the description hereinafter, only one conveyor belt 15 is being considered, even though, at this point, several conveyor belts could be used. Likewise, instead of the two rollers 17, 18, it is possible to use more rollers, for example three or four rollers, should this be advantageous for guiding the conveyor belt 15. Also, instead of the rollers, the use of different guiding or driving elements is also possible such as, for example, wheels, in order to move the conveyor belt 15 in a circulating manner.

Optionally, it is also possible to move the roller 18 up and down, as shown by the double arrow A in FIG. 1, in order to be able to quickly bridge the distance between the conveyor belt 15 and the sheet stack 12 for the accommodation of a sheet 13. During a corresponding movement of the roller 18, a lower side of the conveyor belt 15 performs a pivoting movement in the direction of the stack of sheets and away therefrom. If several conveyor belts are provided next to each other, it may be sufficient to move only one of them over a corresponding roller. Of course, it is also possible to move both rollers 17 and 18 up and down as in FIG. 1, this causing the lower side of the conveyor belt 15 to perform a linear movement toward the sheet stack 12 and away therefrom. The function of the movement will be explained in greater detail hereinafter.

In the figures, the vacuum source 5 is shown as a radial blower 5; however, it should be noted that the vacuum source itself may also be configured as any suction blower of any suitable design. In particular, the vacuum source 5 may be a central negative pressure/positive pressure source that comprises a blower or a compressor, whereby, on the one side of the negative pressure source/positive pressure source, a vacuum is generated for use at various locations of a machine or production line, and whereby, on the other side thereof, a positive pressure is generated for use at another suitable location of the machine or production line, as, for example, considering the aforementioned air knife. The arrows in FIG. 1 show the air current generated by the vacuum source 5.

In accordance with FIG. 1, the conveying arrangement 3 and the vacuum source 5 communicate with each other via an air line 7. On its one end or on the blower end 20, the air line 7 is connected to the vacuum source 5. The other end or suction end 22 is located adjacent to the porous conveyor belt 15. The shape and size of the suction cross-section of the suction end 22 are adapted to the size of the porous conveyor belt 15. Inasmuch as the conveyor belt 15 is porous, the air may enter through the conveyor belt 15 into the suction end 22 of the air line 7 and exit at the blower end 20. A main flap 24 and a constant pressure controller 9 are provided in the air line 7. The main flap 24 is located downstream of the constant pressure controller 9. The main flap 24 is suitable to completely separate the air line 7 from the vacuum source 15 or to provide a rough control of the flow through the air line 7.

A passage opening 27 is provided in one exterior wall of the air line 7, said passage opening creating a connection between the interior of the air line 7 and the environment. The constant pressure controller 9 comprises a valve element 32 in the form of a valve flap and a spring 34. The valve element 32 is arranged in such a manner that it can be moved between a sealing position and a position clearing the passage opening 27. The spring is arranged in such a manner that it biases the valve element in the position sealing the passage opening 27. Spring 34 may be a pressure spring or a tension spring. Furthermore, spring 34 may display a spring characteristic that is variable along the spring path. Thus, the constant pressure controller 9 establishes a connection between the interior of the air line 7 and its environment as soon as a pressure differential between the vacuum in the air line 7 and the atmospheric pressure of the environment reaches a specific threshold value. This threshold value may be adjusted by the operator to match different sheet stacks 12.

The constant pressure controller 9 shown in the figures is of the type that comprises a valve flap. However, it should be noted that the constant pressure controller 9 may also comprise a sliding valve having an opening cross-section that may increase along its opening path, either proportionally or not proportionally. Furthermore, other suitable valves may be taken into consideration for the constant pressure controller 9 such as, for example, diaphragm valves or ball check valves.

FIG. 2 shows an alternative embodiment of a sheet conveying device 1 that is different from the above-described device in view of the design of the air line 7. In this embodiment, the air line 7 comprises a main line 28 and a plurality of branch lines 30. The main line 28 is connected to the vacuum source 5, on the one side, and to the branch lines 30, on the other side. Each of the branch lines 30 leads to a separate conveying arrangement 3, each of said arrangements being allocated a sheet stack 12, for example. Each branch line 30 contains a main flap 24 that can completely separate the associate branch line 30 from the main line 28 or can roughly control a flow. Upstream of each main flap 24, a passage opening 27 is provided in each branch line 30, each of said branch lines being associated with a constant pressure controller 9 as described above. In this embodiment, it is optionally possible to move respectively one of the branch lines 30 together with the respective conveying arrangement 3 toward the sheet support 11 and away therefrom, as is shown by the double arrow B in FIG. 2. The connection to the main line 28 is maintained by suitable means. The joint movement of the branch line 30 with the conveying arrangement 3 ensures that the suction conditions in the region of the conveyor belt 15 are not changed by the movement.

The second embodiment is used to operate several conveying arrangements 3 on a vacuum source 5. This means that the individual conveying arrangements 3 can be operated with different flow rates and negative pressures, respectively.

FIG. 3 shows another embodiment of a sheet conveying device 1 that has essentially the same basic design as the sheet conveying device 1 in accordance with FIG. 1. However, this device is different in view of the constant pressure controller 9, on the one hand, and in view of the provision of at least one optional suction finger 42 as part of the conveying arrangement 3 that can be moved toward the sheet support 11 and away therefrom, on the other hand, as is shown by the double arrow C. For example, a sheet 13 can be lifted off the sheet stack 12 on the sheet support 11 and be transported to the conveyor belt 15 by the suction fingers 42, as will be explained in greater detail hereinafter. The other features and parts of the sheet conveying device 1 of the embodiment in accordance with FIG. 3 are configured as those in FIG. 1. Therefore, no description is provided in order to avoid repetitions.

As an alternative to the at least one suction finger 42 or, also in addition thereto, it is possible to configure the conveying arrangement 3 or parts thereof so as to be movable toward the sheet support 11 or away therefrom. This is indicated by the box 60 in FIG. 3 and by the double arrows D and E, with the double arrow D indicating a linear movement and the double arrow E indicating a pivoting movement. For example, the part of the conveying device 3 that is moved toward the sheet stack 12 may be the porous conveyor belt 15. To do so, it is possible, for example, to move at least one of the rollers 17, 18 toward the sheet stack 12 and away therefrom. Alternatively, it is also possible for an actuating element extending across the conveyor belt 15 to push said conveyor belt 15 in the direction of the sheet stack 12. Another possibility of providing a movement of the conveyor belt 15 is that the suction end 22 of the air line 7 abutting against the conveyor belt 15 is configured so as to be movable in the direction of the sheet stack 12. As a result of such a movement, the conveyor belt 15 is pushed, together with the suction end 22, in the direction of the sheet stack 12.

In accordance with FIG. 3, the constant pressure controller 9 comprises a pressure sensor 36, a control device 38 and a controllably driven pressure control valve 40. In FIG. 3, the pressure control valve 40 is shown as a flap valve; however, it may also be a sliding valve or have another valve design. The pressure, control valve 40 may be driven, for example, by piezo-electric or electromagnetic means. The pressure sensor 36 is suitable to sense the air pressure inside the air line 7 and to output a corresponding signal to the control device 38. The control device 38 is suitable to process the signal output by the pressure sensor 36 and to activate the controllably driven pressure control valve 40 in response to this signal, i.e., to more or less open said valve in order to adjust a substantially constant pressure in the air line 7.

The at least one optional suction finger 42 is arranged adjacent to the conveyor belt and connected with the vacuum source 5 or another suitable vacuum source. The at least one optional suction finger 42 can be moved toward the sheet support 11 and away therefrom. Preferably, suction fingers 42 are arranged to the right and to the left of the conveyor belt 15. Whenever a plurality of conveyor belts are arranged adjacent to each other, the suction fingers 42 can be arranged not only to the right and to the left of the conveying arrangement 3 but also between the individual conveyor belts. However, it is also possible to provide only one suction finger 42—in this instance, preferably between two conveyor belts. In its position lowered relative to the sheet stack 12, the at least one suction finger 42 is connected to the vacuum source 5. However, in its lifted-off position, said suction finger is separated from the vacuum source. The at least one optional suction finger 42 can thus first attract a sheet 13 by suction, then lift said sheet to the conveying arrangement 3 and release said sheet there.

It is also conceivable to move the entire sheet conveying device 1 in the direction of the sheet support 11 and away therefrom. The movable sheet conveying device 1 or its movable part (e.g., conveyor belt 15, suction finger 42, . . . ) can either be moved in linear direction or be pivoted. The distance over which the sheet conveying device 1 or one of its parts can be moved in the direction of the sheet support 11 ranges between 5 and 20 mm.

Hereinafter follows a description of the operation of different sheet conveying devices 1. Arranged on the sheet support 11 is a sheet stack 12 of individual sheets 13, said stack being ready for transport by the sheet conveying device 1. The vacuum source 5. is in operation and generates a vacuum. The main flap 24 is opened, and thus a certain vacuum exists in the air line 7. This initial vacuum, i.e., the pressure differential with respect to the environment, is essentially a function of the suction power of the blower and of the flow resistance of the porous conveyor belt. As a rule, the initial vacuum is set so as to have the constant pressure controller 9 close the passage opening 27.

When a sheet 13 comes close to the porous conveyor belt 15, said sheet is drawn by suction—as of a certain distance—toward the conveyor belt 15 depending on the vacuum in the air line 7 and on the properties of the material of said sheet. By appropriately lifting the sheet support 11 or also appropriately moving a part of the sheet conveying device 1, the sheet can come close to the conveyor belt, as mentioned above. For example, when the sheet 13 comes close to the porous conveyor belt 15 due to a movement of the roller 18 toward the sheet stack 12, said sheet is attracted, and then the roller 18 can again be moved away from the sheet stack 12.

When the sheet 13 has been sucked against the porous conveyor belt 15, said belt is covered by the attracted sheet 13, and the pressure in the air line 7 drops, meaning that the vacuum becomes stronger. Now, there is the risk that the sheet 3 is sucked too strongly against the porous conveyor belt 15 because the vacuum source 5 continues to draw in air through the air line 7.

When a preset pressure threshold value is reached, however, the constant pressure controller 9 opens and establishes the connection between the interior of the air line 7 and the environment, so that the vacuum source 5 will now also suck air in from the environment. This prevents the sheet 13 from being sucked too strongly against the conveyor belt 15. As the vacuum becomes stronger, the constant pressure controller 9 opens wider. If the vacuum becomes weaker, the constant pressure controller 9 opens less widely. Consequently, the vacuum in the air line 7 is maintained essentially constant. Essentially, in so doing, a pressure deviation from the set vacuum of 10%, and preferably of 5%, is to be possible. The movement of a part of the sheet conveying device 1 such as, for example, a roller 17, 18 may be coupled with the opening of the constant pressure control valve, because said control valve signals a safe attraction by suction of a sheet from the stack.

When the sheet 13 has been attracted by suction, the movement of the conveyor belt 15 is started in order to transport the sheet away. When this happens, the suction end 22 of the air line 7 that is being blocked by the sheet 13 is gradually cleared again. Thus, the vacuum source 5 can again suck in more air through the porous conveyor belt 15; and, consequently the vacuum in the air line 7 is reduced. This has the effect that the constant pressure controller 9 again closes gradually until said controller is completely closed when the pressure threshold value is not reached.

As will be obvious to the person skilled in the art, the use of the constant pressure controller 9 permits the adjustment of a relatively high initial vacuum, because damage to a sheet 13 due to an excessive pressure drop in the air line 7 after the attraction of the sheet 13 by suction is prevented. Consequently, the spectrum of sheets that can be attracted without being damaged can be expanded considerably.

In the embodiment of FIG. 2, a vacuum source 5 is used to provide a vacuum for several conveying arrangements 3. Other than that, the operation is substantially the same as has just been described in the case of the embodiment of FIG. 1. However, different branch lines 30 can be shut off completely or activated only partially by their associate main flap. As a result of this, for example, a negative pressure loss is prevented when a conveying arrangement 3 is stopped. The respective constant pressure controller 9 in a branch line 30 controls the suction pressure that is ideal for the sheets 13 that are to be conveyed by the corresponding conveying arrangement 3. For an initial pickup of a sheet, the branch line 30, together with the sheet conveying arrangement 3, can be moved toward the sheet stack 12 and away from said stack. After, or also during, a movement of the branch line 30 and the sheet conveying arrangement 3, the conveyor belt 15 is operated in a circulating manner in order to transport away an attracted sheet. If the porous conveyor belt 15 or a part thereof is lowered in the direction of the sheet stack 12 in order to lift off the uppermost sheet 13, the suction, lifting and transporting steps of the sheet 13 off the sheet stack 12 may essentially take place at the same time. To do so, the conveyor belt 15 is usually driven intermittently.

Hereinafter, the operation of the embodiment of the sheet conveying device 1 shown in FIG. 3 will be described, said device comprising at least one suction finger 42 as a part of the conveying arrangement that can be moved toward the sheet support and away therefrom.

As is the case in the other embodiments, a sheet stack 12 is provided under the conveying arrangement 3. The conveyor belt 15 is initially stopped. For an initial pickup of an uppermost sheet 13 of the stack of sheets, the suction fingers 42 are moved in the direction of the sheet stack 12, and the uppermost sheet 13 is attracted by suction by the suction fingers 42. Subsequently, the suction fingers 42 are again moved away from the sheet stack 12, as a result of which the uppermost sheet enters the region of the conveyor belt 15 and is attracted by suction by said conveyor belt. Then, the suction fingers 42 release the sheet, and the conveyor belt 15 is moved in a circulating manner in order to transport away the sheet 13.

While the suction fingers 42 are lowered near the uppermost sheet 13, they are automatically connected to the vacuum source 5, and the conveyor belt 15 is stopped. While the suction fingers 42 are being lifted and before the conveyor belt is being moved in a circulating manner, the suction fingers are again automatically separated from the vacuum source 5.

Alternatively or also additionally, it is also possible for at least a movable part of the conveying arrangement 3 to be lowered by means of a linear or pivoting movement into the vicinity of the uppermost sheet 13 of the sheet stack 12. As described above, this part may be, for example, the conveyor belt 15 or also the entire sheet conveying device 1. After drawing the uppermost sheet 13 against the conveyor belt 15 by means of a vacuum, the movable part is moved with the sheet 13 away from the sheet stack 12 in upward direction and transported away with the aid of the conveyor belt 15.

In so far as the porous conveyor belt 15 (optionally by itself or also in combination with other elements) is lowered in the direction of the sheet stack 12 in order to lift off the uppermost sheet 13, the attraction by suction and the lifting of the sheet 13 off the sheet stack 12 takes place essentially at the same time. In so doing, the conveyor belt 15 is usually intermittently driven in such a manner that it is not driven in the lowered position.

The invention has been described with reference to preferred embodiments, whereby the individual features of the described embodiments can be freely combined with each other and/or interchanged, provided that they are compatible. Numerous modifications and designs are possible and obvious to the person skilled in the art, without departing from the idea of the invention.

In particular, optional elements of the sheet separating device 1 have been discussed, said elements being movable toward the sheet stack 12 and away therefrom. However, elements other than those that have been mentioned can be provided for a movement toward the sheet stack 12 and away therefrom. For example, the conveyor belt 15 could display a certain elasticity, and an actuation element extending in a direction transverse to the conveyor belt 15 could be provided, said actuation element moving the lower part of the conveyor belt 15 (between the rollers 17, 18) in the direction of the sheet stack 12 and away therefrom. It is also possible for the suction end 22 of the air line 7, said suction end abutting against the conveyor belt 15, to be configured so as to be movable in the direction of the sheet stack 12. With such an embodiment, for example, it is possible to move the conveyor belt 15, together with the suction end 22, in order to maintain the suction geometry consistent in the region of the conveyor belt. It is also possible for the entire sheet separating device 1 to be movable in the direction of the sheet support 11 and away therefrom. The movable sheet separating device 1 or its movable parts (e.g., the conveyor belt 15, the suction fingers 42, . . . ) can be moved either in linear direction or can be pivoted. The distance along which the sheet separating device 1 or its part can move in the direction of the sheet support 11 ranges between 5 and 20 mm, for example. 

1. Sheet conveying device comprising: at least one conveying arrangement with a porous conveyor belt that can be moved in a circulating manner; at least one vacuum source; at least one air line whose one end is connected with the vacuum source and whose other end abuts against the porous conveyor belt; at least one constant pressure controller provided in the air line in order maintain a substantially constant pressure differential between the interior space of the air line and its environment, wherein the air line comprises a main line that is connected, at its one end, to the vacuum source and comprising a plurality of branch lines, each of the plurality of branch lines leading to the conveying arrangement.
 2. Sheet conveying device as in claim 1, with the conveyor belt being arranged so as to circulate around at least two rollers.
 3. Sheet conveying device as in claim 1, with the air line containing downstream of the constant pressure controller a main flap in order to partially or completely seal the air line with respect to the vacuum source.
 4. (canceled)
 5. (canceled)
 6. Sheet conveying device as in Claim 4, with one constant pressure controller being arranged in the region of each branch line.
 7. Sheet conveying device as in claim 4, with the main flap being arranged so as to shut off the branch line with respect to the main line.
 8. Sheet conveying device as in claim 7, with the constant pressure controller being arranged between the main flap and the free end of the branch line.
 9. Sheet conveying device claim 1, with the constant pressure controller being in an open position when a pressure differential between the interior space of the air line and its environment is greater than a threshold value.
 10. Sheet conveying device claim 1, with the constant pressure controller comprising a valve element and a spring that holds the valve element in a closed position.
 11. Sheet conveying device as in claim 10, with the spring displaying a spring characteristic that is variable along the spring path.
 12. Sheet conveying device as in claim 10, with the spring bias being adjustable.
 13. Sheet conveying device as in claim 1, said device further comprising a pressure sensor.
 14. Sheet conveying device as in claim 4, said device comprising at least one pressure sensor that is arranged in a branch line, respectively.
 15. Sheet conveying device as in claim 13, said device further comprising a controllably driven pressure control valve and a control device that communicates with the pressure sensor and the pressure control valve in order to control the actuation of the pressure control valve.
 16. Sheet conveying device as in claim 1, with the constant pressure controller comprising a flap valve.
 17. Sheet conveying device as in claim 1, with the constant pressure controller comprising a sliding valve.
 18. Sheet conveying device as in claim 15, with the opening cross-section of the sliding valve not increasing proportionally along its opening path.
 19. Sheet conveying device as in claim 1, said device being arranged in a sheet separating device.
 20. Sheet conveying device as in claim 1, said device being arranged in a feeder of a printing machine.
 21. The sheet conveying device claim 1 further comprising a sheet support for the accommodation of a stack of sheets, with at least one part of the sheet conveying device being movable toward the sheet support and away therefrom in order to be able to lift a sheet off the stack of sheets on the sheet support.
 22. Sheet conveying device as in claim 21, with the sheet support being height-adjustable.
 23. Sheet conveying device as in claim 21, with the part of the sheet conveying device being the at least one conveyor belt.
 24. Sheet conveving device as in claim 23, with the part of the sheet conveying device being at least one of the rollers.
 25. Sheet conveying device as in claim 21, with the part of the conveying arrangement being at least one suction finger connected to the vacuum source.
 26. Sheet conveying device as in claim 25, with the suction finger being preferably separated from the vacuum source when said suction finger is in a position away from the sheet support.
 27. Sheet conveying device as in claim 26, with the connection of the suction finger with the vacuum source being preferably open toward the environment when said suction finger is in its position away from the sheet support.
 28. Sheet conveying in device as in claim 21, with the sheet conveying device being movable, as a whole, toward the sheet support and away therefrom.
 29. Sheet conveying device as in claim 21, with the part of the conveying arrangement being moved in linear direction.
 30. Sheet conveying device as in claim 21, with the part of the conveying arrangement being pivotable.
 31. Method for separating sheets in a sheet conveying device, comprising the following steps: providing a stack of sheets below a sheet conveying device operating with a vacuum; attracting the uppermost sheet of the stack of sheets by means of a vacuum; limiting the level of the vacuum with which the sheet is being attracted to a prespecified value following the attraction of the sheet, that is, via a constant pressure control valve inside the sheet separating device.
 32. Method as in claim 31, wherein at least one part of the sheet conveying device is moved into the vicinity of the uppermost sheet of the stack of sheets before attracting the uppermost sheet by vacuum, and moving the at least one part of the sheet conveying device away from the stack of sheets after attracting the uppermost sheet.
 33. Method as in claim 32, wherein the step of moving at least one part of the sheet conveying device comprises the step of moving at least one suction finger to which a vacuum is being applied.
 34. Method as in claim 32, wherein, following the step of moving the part of the sheet conveying device away from the stack of sheets, the vacuum on the suction finger is shut off.
 35. Method as in claim 32, wherein the step of moving at least one part of the sheet conveying device comprises the step of moving at least one porous conveyor belt.
 36. Method as in claim 32, wherein the step of moving at least one part of the sheet conveying device comprises the step of moving the entire sheet conveying device.
 37. Method as in claim 32, wherein the steps of moving the part of the sheet conveying device away from the stack of sheets and of transporting away the sheet occur at the same time.
 38. Method as in claim 32, wherein the step of moving at least one part of the sheet conveying device comprises a linear movement.
 39. Method as in claim 32, wherein the step of moving at least one part of the sheet conveying device comprises a pivoting movement. 